(a) Field of the Invention
This invention relates to a stayed connection for a wind turbine. More particularly it relates to a stayed connection for a wind turbine where the wind turbine tower comprises an upper, pipe-shaped tower portion and a lower tower portion, and where the upper pipe-shaped tower portion is connected to the lower tower portion by means of a stayed connection.
(b) Description of Related Art
The upper portion of a wind turbine is formed like a slender structure to avoid collision with the wind turbine sails. From land based wind turbines is known that this slender structure which is generally made of a pipe, is connected to a foundation in the ground.
When wind turbines are positioned offshore and more often in relatively deep water, using a pipe extending from the wind turbine foundation on the sea bed and up to the wind turbine machinery housing, is not practical.
The load bearing structure in known wind turbines positioned offshore therefore often comprise a pipe shaped upper portion and a lower portion, where the lower portion may be constituted by such as a trussed structure.
The transition between the upper slender portion of the tower and the lower portion is often constituted by a relatively heavy and complicated structure. The reason for this is among other things the use of design principles known from offshore-based oil production equipment. Such equipment is dimensioned for considerable wave forces and to be able to handle heavy equipment, and also for personnel to reside on the structure at all times.
In wind turbine installations it is common, also offshore, that the wind forces decide the structural design.
Transitions of this prior art are relatively flexible. They have to be adapted so that the tower natural period is short enough for the relevant wind turbine. This contributes to a further increase in tower weight.
It is a known problem that in stayed structures so-called secondary forces exists due to the shape and relative dimensions of the structure. Considerable bending stresses may for example be set up in a first stay due to deflection in a second stay when the first stay is welded to the second stay.
Such secondary forces cause the stay dimension to be increased and contributes thus to increased weight and cost.